Solar cells, for example photovoltaic cells (PVCs), have been used for many years to generate electrical energy from sunlight. Hereafter, the terms “solar cells” and “PVCs” will be used interchangeably and refer to cells that generate electrical power from exposure to light. Solar panels, which typically include many individual cells, have been deployed in space and terrestrial applications.
Terrestrial photovoltaic cells may be exposed to “multiple” sun sources using mirrors, reflectors, and/or lenses that concentrate sunlight into a smaller area, which results in higher radiation energy per square unit of area. Such concentration is desirable to generate higher current per cell. This concentrated level of energy generates high levels of heat that places stresses on the internal structures of the PVC as well as electrical connections and mechanical attachment points. Temperature gradients often develop between adjacent portions of the PVC.
Over time, these elevated temperatures and temperature gradients degrade the performance of PVCs and can trigger failures in the PVC, electrical connections or mechanical attachment points. Understanding the conditions under which PVCs fail enables engineers to develop solutions to mediate design problems within the PVCs and associated structures. Stress testing can assist engineering in developing failure rate metrics useful for system integrators that use PVCs in commercial applications.
Accordingly, test equipment and technologies for terrestrial photovoltaic cells are designed to test PVCs not only by approximating the incident light and environmental conditions likely to be seen by the PVCs, but also by thermally stressing the PVCs to determine the long term effects of thermal stresses on the PVCs. These methods can involve creating higher thermal stresses and sharper temperature gradients than typically would be seen in commercial applications. Creating these thermal stresses allow characterization of the PVCs in comparatively shorter periods of time.
Recreating the thermal stresses on the PVC can be accomplished in various ways. Current tests include exposing the PVCs to concentrated sunlight for extended periods of time, placing PVCs in thermal cycling chambers to simulate different thermal conditions, and applying electrical currents to stress the PVCs and electrical connections.
Many thermal tests take comparatively long periods of time to perform. Thermal test methods include placing the PVC to be tested in a controlled temperature environment, such as a thermal cycle chamber where inside the chamber the ambient temperature can be controlled. The ambient temperature is then cycled to different temperatures for varying periods of time, and then the performance of the PVC is measured to determine how the PVC was affected. Generally, it takes some time for all the components to equalize with the internal ambient temperature using a thermal cycle chamber, and therefore cycle times for some tests can be fairly long, lasting from minutes to hours for each cycle.
Moreover, thermal cycle chambers typically are not representative of operating conditions in the field. Thermal cycle chambers convectively heat or cool the PVCs test samples evenly over a relatively long period of time. In contrast, the field temperature stresses typically occur much faster. Also the distribution of heat in the field will generally tend to be non-uniform across the entire PVC assembly. For example, in a thermal cycle chamber, the temperature typically is consistent from the front to the rear of the solar cell and at the mechanical and electrical interconnections. In the field, however, sunlight heats the front of the PVC whereas the rear of the PVC is typically attached to a heat sink structure, creating a temperature gradient from the front of the PVC to the rear of the PVC. Also, in the field, the mechanical and electrical connections often receive relatively little or no heating from sunlight, but considerable heating from convection, heat conduction, or electrical current passing through them.
Another thermal test method is the dark forward thermal cycle. Often performed in a thermal cycle chamber, the dark forward thermal cycle involves forward biasing the PVC to generate current through the PVC. The generated current simulates approximately the amount of current that would be produced by illuminating the PVC with sunlight. Using the dark forward thermal cycle method, it is also possible to force more current through the PVC than would be possible using illumination alone.
Each of these test methods requires the application of a unique set of parameters, to a multitude of variously sized PVCs, all with a consistent application of the testing parameters to ensure reliability and accuracy. More specifically, many of the previously practiced test methods resulted in damage to a large number of tested PVCs as a result of using threaded vacuum ports (having metal burrs stick into the PVC) and using blade probes (causing scrub and damage to the PVC when probing). Heretofore, transitioning between tests of different sized PVCs required varying degrees of system reconfiguration that was time consuming, inefficient, and damaged the PVCs.
Advantages over the prior art are herewith provided in the following disclosure.